1. Field of the Invention
The present invention relates to a novel amphiphilic substance, and a drug delivery system and a molecular imaging system using the novel amphiphilic substance.
2. Disclosure of the Related Art
As described in Japanese Unexamined Patent Publication No. 2005-172522, in recent years, there has been a growing interest in nanotechnology, and new functional materials utilizing the characteristics inherent in nanosized substances have been developed. These new functional materials can be used in various fields such as energy, electronics, and medical and pharmaceutical fields. Among such various fields, nanotechnology is attracting attention in the field of detection of substances in biological samples and in-vivo imaging. Particularly, in the medical and pharmaceutical field, liposomes and the like which are nanoparticles composed of phospholipid are used as carriers for drug delivery system (DDS).
As described in Japanese Unexamined Patent Publication No. 2005-220045, in the medical and pharmaceutical field, it is desired that changes in the form and function of organs or tissues caused by diseases in a living body are promptly and accurately detected by a simple method at early stages of the diseases to early diagnose and treat the diseases. Particularly, in order to early diagnose and treat cancer, it is essential to detect a small diseased part and to determine the size of the diseased part at its early stage. Examples of a method for early diagnosis include endoscopic examination and diagnostic imaging such as X-ray imaging, MRI, and ultrasonic imaging. In a case where a radioactive indicator is used, the lifetime of the indicator is limited due to its half-life. In addition, in this case, a diagnostic apparatus is very expensive.
On the other hand, diagnostic imaging can also be carried out using a fluorescence indicator or a near-infrared indicator. Such a diagnosis method does not impose strict limitations on its indicator's own lifetime, and a diagnostic apparatus for this method is not very expensive as compared to a radiodiagnostic apparatus. In addition, such optical diagnosis is noninvasive to a living body. For example, autofluorescence observation via endoscope is in practical use, which utilizes the fact that the autofluorescence of tumor cells is weaker than that of normal cells (excitation: 450 nm, emission: 520 nm).
As a near-infrared diagnosis method, near-infrared fluorescence imaging, in which a near-infrared fluorochrome is allowed to accumulate in a tumor region thereby enabling imaging of the tumor region, is also attracting attention. In this method, a compound that can emit fluorescence in the near-infrared region by irradiation with excitation light is administered as a contrast agent into a living body, and then the body is externally irradiated with excitation light having a near-infrared wavelength to detect fluorescence emitted from the fluorescent contrast agent accumulating in a tumor region. In this way, a diseased region is determined. As such a contrast agent, a nanoparticle, such as a liposome, having an indocyanine green derivative encapsulated therein has been proposed (see Japanese Unexamined Patent Publication No. 2005-220045).
On the other hand, peptide nanoparticles having higher biocompatibility have also been known (see Journal of Controlled Release 50 (1998) 205-214, Journal of Controlled Release 51 (1998) 241-248, Journal of Colloid Interface Science 280 (2004) 506-510, and Journal of American Chemical Society 2005, 127, 12423-12428).